Fan curves—also called P-Q curves or pressure-volume curves—are the most important technical document in thermal system design. Yet they are routinely misread, or not read at all. Engineers frequently select fans based on "maximum CFM" specifications in datasheet headlines, then find that the actual delivered airflow in their system is 40–60% lower than expected.

This guide explains how to read fan curves correctly, how to construct a system impedance curve, how to find the operating point, and what to do when your initial selection does not meet your cooling target.

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What a Fan Curve Actually Shows

A fan's P-Q (Pressure-Volume) curve plots the relationship between: - X-axis: Volumetric airflow (CFM or m³/h) - Y-axis: Static pressure developed by the fan (Pa or inH₂O)

At the left end of the curve (zero flow), the fan develops its maximum static pressure—but moves no air. This is the stall point or shutoff pressure.

At the right end (zero pressure), the fan moves its maximum volume of air—but develops no pressure. This is the free delivery point, also called free air flow or open flow.

Every real installation falls somewhere between these extremes. The fan must develop enough pressure to overcome the system resistance while delivering the required airflow.

The maximum power point is typically at 50–70% of free air flow. Fan motors are sized for this peak power condition. Operating at free delivery or deep into stall both create problems: free delivery risks motor overload; stall causes flow separation, vibration, and accelerated bearing wear.

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Constructing a System Impedance Curve

The system impedance curve describes how much pressure drop your cooling system creates at various flow rates. It follows the relationship:

ΔP_system = k × Q²

Where: - ΔP_system = system pressure drop (Pa or inH₂O) - k = system resistance coefficient (determined by testing or calculation) - Q = volumetric flow rate

To construct the curve, you need the pressure drop of each component in the airflow path:

Typical components in an industrial cabinet cooling path: 1. Inlet filter (polyester foam): 3–8 Pa at design flow, 15–25 Pa when 50% clogged 2. Inlet screen or grille: 0.5–2 Pa 3. Heatsink fin array: 2–10 Pa depending on fin density and flow velocity 4. Cable management bundles, PCB card guides: 1–5 Pa 5. Exhaust filter: 3–8 Pa

Example calculation for a 120mm filter fan installation:

Assume clean filter total system resistance = 10 Pa at 100 CFM (1.7 m³/min). Then k = 10 / (1.7)² = 3.47 Pa/(m³/min)²

To construct the system curve, calculate ΔP at multiple flow rates:

| Flow (m³/min) | System ΔP (Pa) | | 0.5 | 0.87 | | 1.0 | 3.47 | | 1.5 | 7.8 | | 1.7 | 10.0 (design point) | | 2.0 | 13.9 | | 2.5 | 21.7 |

Plot this curve on the same graph as the fan's P-Q curve. The intersection is the operating point.

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Reading the Operating Point

Case 1: Adequate airflow margin

If the operating point is at or above your required airflow (calculated from thermal analysis), your fan selection is confirmed. A 20–30% airflow margin above the minimum required is good practice, accounting for: - Filter clogging over service life - Airflow non-uniformity (ducting losses, recirculation) - Fan-to-fan production variance (±5–10% in airflow) - Temperature-dependent density effects (hot air is less dense, reducing mass flow)

Case 2: Insufficient airflow at operating point

Options: 1. Select a fan with higher static pressure at the required flow rate — not necessarily higher free-air flow 2. Reduce system resistance by using a coarser filter media, enlarging inlet/outlet apertures, or removing flow obstructions 3. Use two fans in series — doubles static pressure at the same flow; useful when system resistance is the constraint 4. Use two fans in parallel — doubles flow at the same static pressure; useful when more volume is needed

Series vs. parallel configuration:

| Configuration | Effect on P-Q Curve | When to Use | | Two fans in series | Double the pressure axis | High-resistance systems (dense heatsinks, long ducts) | | Two fans in parallel | Double the flow axis | Low-resistance systems needing more volume |

For most industrial cabinet filter fan applications (low system resistance, short air path), parallel fans are more effective than series configurations.

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The Fan Curve in Practice: A DCFC Thermal Design Example

Application: 150 kW DC fast charger, 8 kW heat rejection required Constraints: 40°C maximum ambient, 55°C maximum internal temperature Required airflow: 12 m³/min (calculated from heat balance) System resistance: 12 Pa at 12 m³/min (includes inlet filter, heatsink, exhaust filter)

Initial candidate: SXDOOL SXDE28080BTM (280×80mm EC fan) - Free air flow: 24.5 m³/min (850 CFM) - Maximum static pressure: 300 Pa (1.20 inH₂O)

System curve at 12 Pa: Q = √(12/k). If k = 0.083 Pa/(m³/min)², then the operating point is approximately 21 m³/min at 37 Pa.

Result: Delivered airflow (21 m³/min) exceeds requirement (12 m³/min) by 75% margin. This allows the EC fan's PWM control to reduce speed to approximately 70–75%, maintaining thermal compliance while reducing acoustic output from 68 dB(A) to approximately 59 dB(A).

This is the advantage of EC fan speed control: the fan is sized with margin for high-ambient-temperature conditions, then throttled during normal operation.

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Common Fan Curve Mistakes and How to Avoid Them

Mistake 1: Selecting based on free-air CFM A fan with 300 CFM free-air flow may deliver only 180 CFM in your system. Always find the operating point on the P-Q curve.

Mistake 2: Ignoring filter pressure drop A clean polyester filter adds 3–8 Pa. A partially clogged filter may add 20 Pa. Size the fan for the clogged condition, not the clean condition.

Mistake 3: Not accounting for temperature on air density At 50°C, air density is approximately 1.07 kg/m³ vs. 1.20 kg/m³ at 20°C. This reduces mass flow (and heat removal capacity) by approximately 11% compared to a room-temperature calculation.

Mistake 4: Using stall region operation Operating a fan in the left portion of its P-Q curve (high pressure, low flow) causes unstable flow, increased noise, vibration, and accelerated bearing wear. Ensure the system curve intersection is in the stable operating region (typically right of the peak pressure point).

Mistake 5: Neglecting back-pressure from adjacent parallel fans In multi-fan arrays, fans share the outlet plenum. Each fan's backpressure from adjacent units reduces the effective driving pressure. Account for mutual interference in the system curve calculation.

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Requesting P-Q Data from SXDOOL

SXDOOL provides full P-Q curve data in both graphical and tabular formats for all standard models. For custom OEM applications requiring non-standard impeller pitch, motor windings, or connector types, we can provide measured P-Q data for prototype samples before production commitment.

Available fan curve data: - Static pressure vs. volumetric flow at rated voltage - Fan curves at 70%, 80%, and 100% speed (EC models) - Power consumption at operating point - Acoustic data (dB(A) at 1 m, A-weighted) at multiple speeds

Contact our engineering team: david@sxdool.com Visit www.sxdool.com | WhatsApp: +86 134 3209 3474

Engineering sample fast-track: 48 hours from order to shipment for standard models.